if israel strikes iranian nuclear sites, he says, american involvement then would depend on how iran reacts. >> if they retaliate by striking american assets then it makes it easier for obama or romney to essentially respond in kind. and perhaps they sink part of the iranian navy, but perhaps they also go after those same nuclear sites that israel damaged, but perhaps did not completely destroy. who knows what would come after that? that's where the danger lies. >> another major challenge, the messy fallout from the seismic "arab awakening" that began nearly two years ago. bringing the rise of elected islamist governments in egypt and elsewhere -- and a raging civil war in syria. here, too, stark rhetoric masks murky differences. romney says he'd do more to arm syria's rebels but has not said the u.s. would do the arming. the obama white house has resisted doing so, for fear heavy weapons would end up with anti-american jihadists or terrorists. vali nasr is dean of the johns hopkins school of advanced international studies. >> the differences between the candidates at the moment do n